1. Field of the Invention
The present invention relates to an image forming apparatus for forming a multi-colored image by superimposing a plurality of plane images. The invention also relates to a control method for the above type of image forming apparatus.
2. Description of the Related Art
As color-image forming apparatuses for printing color image data, laser beam printers (LBPs) are known. In LBPs, scanning is performed on a photosensitive member by reflecting a laser beam on a rotating polygonal member, and latent images, each corresponding to one line of a plane image, are sequentially formed on the photosensitive member. One of the developers (toners), such as magenta (M), cyan (C), yellow (Y), and black (BK), is then attached to the latent images for a plurality of lines (latent images for one frame), thereby forming a plane image for one color. The plane image for one color is transferred to paper fixed on a transfer drum. Then, printing an image for one color is completed. Subsequently, printing operations on the remaining three colors are sequentially performed. Finally, a multi-colored image of four colors is ready to be printed.
Alternatively, another printing method is available. Plane images of the respective four colors formed on a photosensitive member are temporarily superimposed on each other on an intermediate transfer member, and then, the superimposed plane images on the intermediate transfer member are transferred to paper simultaneously.
In the above-described image forming apparatuses, such as LBPs, a plurality of plane images are sequentially superimposed while they are being driven in a sub-scanning direction. More specifically, the photosensitive member, the transfer member, and the intermediate transfer member are driven at a constant velocity in the direction (sub-scanning direction) perpendicular to a main scanning direction. The individual plane images formed on the photosensitive member are transferred to the transfer member or the intermediate transfer member and are superimposed in synchronization with a sub-scanning start signal, which is generated every time the transfer member or the intermediate transfer member is rotated one turn. With this arrangement, positional offsets of the plane images can be reduced.
Alternatively, instead of transferring each plane image for one color to the subsequent stage from the photosensitive member, latent images for four colors may be sequentially formed on the photosensitive member to create the respective plane images for four colors. In this case, the resulting color images formed on the photosensitive member are simultaneously transferred to the subsequent transfer material (paper).
In the aforementioned color-image forming techniques, it is desirable that the individual plane images be superimposed with a minimal amount of positional offset so as to obtain a printed multi-colored image of high quality.
Hitherto, various methods for reducing the amount of the above positional offset have been considered. For example, the number of main-scanning start signals (beam detection (BD) signals) which are obtained while the photosensitive member is rotated one turn is adjusted to be an integer (FIG. 18B). With this arrangement, the operation of a motor for driving the photosensitive member is synchronized with the operation of a scanner motor for driving the main scanning operation.
The aforementioned method is discussed in detail below with reference to FIGS. 18A and 18B. FIG. 18A and 18B schematically illustrate main scanning lines formed on a photosensitive member or an intermediate transfer member of a conventional image forming apparatus.
In FIGS. 18A and 18B, reference numeral 801 indicates an image carrier, such as a photosensitive member or an intermediate transfer member, which will hereinafter be described as a photosensitive member. An ITOP (a signal indicating the top position of a recording sheet) sensor 802 detects a sensor flag (not shown), which is disposed at a predetermined position on a lateral surface of the photosensitive member 801, while the photosensitive member 801 is being rotated one turn, and generates a sub-scanning start signal (ITOP signal).
FIG. 18A illustrates the main scanning lines when the number of main-scanning start signals (BD signals) generated while the photosensitive member 801 is rotated one turn is set to be n+1/2. That is, FIG. 18A illustrates an example in which the individual plane images fail to be precisely superimposed on one another. In FIG. 18A, there are shown positions of the main-scanning recording line signals, i.e., the first line, the second line, the third line, the (n-1)-th line, the n-th line of the first rotation, and the first line and the second line of the second rotation of the photosensitive member 801. FIG. 18A illustrates the main scanning lines up to the third lines of the second rotation of the photosensitive member 801.
FIG. 18A reveals that every time the photosensitive member 801 is rotated one turn, i.e., every time one sub-scanning start signal is generated, the first line of the first rotation of the photosensitive member 801 is offset with respect to the first line of the second rotation by "0.5" lines. Similarly, for every rotation of the photosensitive member 801, such as the third time, the fourth time, . . . , the (n-1)-th time, and the n-th time, "0.5" lines of offsets are generated between the same order of lines of the (n-1)-th rotation and the n-th rotation.
FIG. 18B illustrates the main scanning lines when the number of main-scanning start signals (BD signals) generated while the photosensitive member 801 is rotated one turn is set to be "n". That is, FIG. 18B illustrates an example in which the individual plane images can be precisely superimposed on each other. FIG. 18B illustrates the main scanning lines up to the third lines of the second rotation of the photosensitive member 801.
FIG. 18B shows that even after the photosensitive member 801 has been rotated a few turns, the same order of lines of the respective rotations, for example, the first line of the first rotation and the second line of the second rotation, theoretically match.
In conventional image forming apparatuses, the following methods are known as methods for providing synchronization between the operation of a motor for driving the photosensitive member or the intermediate transfer member and the operation of the scanner motor for driving the main scanning operation.
In a first method, a signal obtained by scaling a BD signal is used as a reference clock for a motor for driving the photosensitive member or the intermediate transfer member. In the second method, the reference clock for a drum motor for driving the photosensitive member or the intermediate transfer member and the reference clock for a scanner motor for driving the main scanning operation are generated in the same oscillator. Examples of the above-described methods are discussed in detail below.
FIG. 19 illustrates a conventional image forming apparatus constructed in accordance with the aforementioned first method.
In FIG. 19, a photosensitive member 901 is rotated by a photosensitive-member driving motor 907 via a driving belt 908. A scanner motor 902 is controlled to operate at a constant velocity by a phase-locked loop (PLL) circuit 910 based on a reference clock supplied from an oscillator 911, thereby driving a polygonal mirror 903. The polygonal mirror 903 deflects on its mirror planes (for example, eight planes) a laser beam applied from a laser 904 and line-scans the photosensitive member 901 in a planar form via a lens 905.
A beam detect (BD) sensor 906 is disposed on a portion free from an image forming region on the scanning lines of a laser beam, and generates a main-scanning start signal (BD signal) synchronized with the operation of the scanner motor 902, i.e., synchronized with every laser scanning line. For every rotation of the polygonal mirror 903, eight BD signals are generated. A PLL circuit 909 controls the photosensitive-member driving motor 907 to operate at a constant velocity by using the BD signal generated from the BD sensor 906 as a reference clock. With this arrangement, the operation of the scanner motor 902 can be synchronized with the operation of the photosensitive-member driving motor 907.
According to the second method, as noted above, the same clock is used as a reference clock for a motor for driving the photosensitive member or the intermediate transfer member and as a reference clock for a scanner motor for driving the main scanning operation.
FIG. 20 illustrates a known image forming apparatus constructed in accordance with the above-described second method.
In FIG. 20, a photosensitive member 1001 is rotated by a photosensitive-member driving motor (drum motor) 1007 via a driving belt 1008. A scanner motor 1002 is controlled by a PLL circuit 1010 to operate at a constant velocity based on a reference clock supplied from an oscillator 1011, thereby driving a polygonal mirror 1003. The polygonal mirror 1003 deflects on its mirror planes a laser beam applied from a laser 1004 and scans the photosensitive member 1001 in a planar form via a lens 1005.
A PLL circuit 1009 controls the photosensitive-member driving motor 1007 to operate at a constant velocity based on the reference clock generated from the oscillator 1011, which is also used for controlling the scanner motor 1002 via the PLL circuit 1010.
As discussed above, the reference clock for the PLL circuit 1009 for controlling the photosensitive-member driving motor 1007 and the reference clock for the PLL circuit 1010 for controlling the scanner motor 1002 are generated from the same oscillator 1011. Thus, the operation of the scanner motor 1002 can be synchronized with the operation of the photosensitive-member driving motor 1007.
According to the above-described first method or second method, the number of main-scanning start signals (BD signals) obtained while the photosensitive member or the intermediate transfer member is rotated one turn and the number of main-scanning recording lines synchronizing with the BD signals are set to be integers, so that the operation of the motor for driving the photosensitive member can be synchronized with the operation of the scanner motor for driving the main scanning operation. Accordingly, even after the photosensitive member or the intermediate transfer member has been rotated any number of turns, no positional offset occurs to the sub-scanning start position, and a plurality of plane images can be superimposed without being offset with respect to one another on the photosensitive member, the intermediate transfer member, or a transfer material (paper), thereby forming a printed image of high quality.
As another example of a method for controlling the sub-scanning start position, phase matching is provided between the main-scanning start signal and the sub-scanning start signal. This makes it possible to fix the sub-scanning start position without needing to set the number of main-scanning start signals (BD signals) obtained while the photosensitive member or the intermediate transfer member is rotated one turn and the number of corresponding main-scanning recording line signals to be an integer. An example of this method is discussed below as a third method.
FIG. 21 illustrates a conventional image forming apparatus constructed in accordance with the third method.
In FIG. 21, a photosensitive member 1101 is rotated by a photosensitive-member driving motor 1107 via a driving belt 1108. A PLL circuit 1109 controls the photosensitive-member driving motor 1107 to operate at a constant velocity based on the reference clock supplied from an oscillator 1114. An ITOP sensor 1115 generates an ITOP signal by causing a sensor flag 1116 to shield the ITOP sensor 1115 from a beam every time the photosensitive member 1101 is rotated one turn. Based on this ITOP signal, the position at which the recording of the first line on the photosensitive member 1101 is started is determined.
A phase matching circuit 1112 provides synchronization so that the reference clock generated from the oscillator 1113 is in phase with the ITOP signal generated from the ITOP sensor 1115. A PLL circuit 1110 controls a scanner motor 1102 to operate at a low speed based on the reference clock in phase with the ITOP signal.
In this manner, phase matching is performed by the phase matching circuit 1112 on the ITOP signal and the reference clock so as to always adjust the rotational phase of the scanner motor 1102 to each ITOP signal. Accordingly, the rotational phase of the polygonal mirror 1103 driven by the scanner motor 1102 is also synchronized with the ITOP signal. This makes it possible to perfectly align, relative to the ITOP signal, the scanning lines on the photosensitive member 1101 formed by applying a laser beam from the laser 1104 via the lens 1105.
FIG. 22 is a schematic diagram illustrating the relationship between the main scanning lines (main-scanning start signal) formed on the photosensitive member of a known image forming apparatus and the ITOP signal (sub-scanning start signal).
In FIG. 22, reference numeral 1601 represents an image carrier, such as a photosensitive member or an intermediate transfer member, which will hereinafter be described as a photosensitive member. An ITOP sensor 1602 detects a sensor flag (not shown), which is disposed on a predetermined position of a lateral surface of the photosensitive member 1601, and generates a sub-scanning start signal (ITOP signal) every time the photosensitive member 1601 is rotated one turn.
The photosensitive member 1601 is rotated one turn while main scanning is performed for "n+(1/2)" (n is an integer) lines. The ITOP sensor 1602 generates a sub-scanning start signal at a predetermined position for each rotation of the photosensitive member 1601. With this configuration, since main scanning is performed for "n+(1/2)" lines for each rotation of the photosensitive member 1601, the first line of the first rotation is offset with respect to the first line of the second rotation by "1/2" lines, as illustrated in FIG. 18A.
However, by virtue of the phase matching circuit 21, such as the one shown in FIG. 21, every time the ITOP signal (sub-scanning start signal) is generated, the main-scanning start signal (the rotational phase of the scanner motor 1102 synchronizing with the BD signal) is controlled to synchronize with the ITOP signal. Thus, the first lines of the respective rotations of the photosensitive member 1601 can be aligned, as shown in FIG. 22, in response to the respective ITOP signals. It is thus possible to match the individual plane images, free from the occurrence of offset, even if the photosensitive member or the intermediate transfer member is rotated any number of turns.
In addition to the aforementioned techniques for preventing positional offsets of plane images, the following technique is also known, as disclosed in Japanese Patent Laid-Open No. 5-191608: the phase difference between the sub-scanning start signal (ITOP signal) and the main-scanning start signal (BD signal) is detected in advance, and the timing of forming each plane image is suitably adjusted in accordance with the detected amount of offset.
According to the above-described techniques, however, positional offsets of plane images can be eliminated if all the environmental conditions around the image forming apparatuses are ideal. In practice, the above-described techniques cannot perfectly overcome the aforementioned problems.
For example, there are slight variations in the rotational speed of each of the photosensitive member, the intermediate transfer member, and the transfer member due to the influence of load variations or a backlash of a driving transfer gear, etc. Accordingly, the actual phase difference between the main-scanning start signal and the sub-scanning start signal deviates from the ideal phase difference. This causes a further color shift even by the use of the aforementioned technique for aligning the positions of the laser scanning lines on the photosensitive member. Such deviations can be reduced to one-half or even less by suppressing load variations in the motor to a minimal level or by improving the precision of a mechanical driving transfer system.
However, if the phase of a sub-scanning start signal for recording each color is generated after the corresponding main-scanning start signal, a full line of offset disadvantageously occurs, albeit only one half or even less of one line is actually generated.
Practically, as noted above, the above-described deviation incurred while a drum (photosensitive drum, an intermediate transfer drum, or a transfer drum) is rotated one turn, can be reduced to one-half or even less of one line with a minimal amount of load variation of a motor or an enhanced mechanical driving transfer system.
Nevertheless, in superimposing a plurality of plane images, for example, of yellow (Y), magenta (M), cyan (C), and black (BK), the following problem may be presented according to the phase of the ITOP signal corresponding to the recording start signal of the first plane image (Y). If the phase of a sub-scanning start signal for recording the final plane image (BK) is generated after the corresponding main-scanning start signal, a full line of offset occurs even if only one half or even less of one line is actually generated. This situation is described in detail below with reference to FIGS. 23A and 23B.
FIGS. 23A and 23B illustrate the image forming timing in a conventional image forming apparatus and also illustrates the case in which the phase of a sub-scanning start signal for recording each color is generated after the corresponding main-scanning start signal.
The sub-scanning start signal corresponding to the first rotation of the photosensitive member is generated, as shown in FIG. 23A, slightly before the generation of the main-scanning start signal 1. Accordingly, the sub-scanning start signal starts to scan the first line in synchronization with the main-scanning start signal 1, and begins to scan the second line in synchronization with the main-scanning start signal 2, and further commences to scan the third line in synchronization with the main-scanning start signal 3, thus sequentially scanning the respective lines on the photosensitive member. That is, the main-scanning start signals 1, 2, and 3 are regarded as the first, the second, and the third lines, respectively. In this manner, a first plane image is formed on the photosensitive member.
On the other hand, the sub-scanning start signal corresponding to the second rotation is generated slightly later than the main-scanning start signal 1, thus failing to recognize the main-scanning start signal 1. The sub-scanning start signal thus unfavorably starts to scan the first line in synchronization with the main-scanning start signal 2, and begins to scan the second line in synchronization with the main-scanning start signal 2, thus sequentially scanning the respective lines on the photosensitive member. That is, the sub-scanning start signal considers the main-scanning start signal 2 as the first line without recognizing the main-scanning start signal 1. Then, the sub-scanning start signal regards the main-scanning start signal 3 as the second line without recognizing the main-scanning start signal 2. In this manner, the second plane image is formed. There is thus generated a maximum of one line of offset between the image-recording start position of the first line and that of the second line (FIG. 23B).
In FIG. 23B, reference numeral 1201 indicates an image carrier, such as a photosensitive member or an intermediate transfer member, which will hereinafter be described as a photosensitive member. An ITOP sensor 1202 causes a sensor flag (not shown) to shield the ITOP sensor 1202 from a beam in accordance with the rotation of the photosensitive member 1201, thereby generating a sub-scanning start signal.
The sub-scanning start signal corresponding to the first rotation is generated slightly before the main-scanning start signal 1, and the sub-scanning start signal corresponding to the second rotation is generated slightly after the main-scanning start line 2. This incurs one line of offset between the first line of the first rotation and the first line of the second rotation.
As discussed above, by causing the sensor flag (not shown) to shield the ITOP sensor 1202 from a beam, the ITOP sensor 1202 generates the sub-scanning start signal in accordance with the rotation of the photosensitive member 1201. The position at which the sub-scanning start signal for the first rotation is generated is slightly before the corresponding main-scanning start signal, while the position at which the sub-scanning start signal for the second rotation Is generated is slightly after the corresponding main-scanning start signal.
FIG. 23B illustrates the pseudo-main-scanning start signals on the photosensitive member 1201. Although the main-scanning start signals 1, 2and 3 for the first rotation and those for the second rotation are completely different, they are shown at the same positions in FIG. 23B. The above-described offsets of the sub-scanning start signals are discussed in detail below with reference to FIG. 24.
FIG. 24 is a timing chart illustrating the image forming timing in a known image forming apparatus and also corresponds to the timing chart shown in FIG. 23A. The same signals as those shown in FIG. 23A are designated with like numbers.
In the conventional image forming apparatus, after a video clock (video CLK) counts "n" times in synchronization with the main-scanning start signal, a memory read signal is generated in the period in which the video CLK is counting "m" times. In synchronization with the memory read signal, the reading of recording data from a memory (not shown) is started, and the data read from the memory is then recorded on the photosensitive member by scanning the respective lines of the member with a laser. The sub-scanning start signal is generated at a predetermined position every time the photosensitive member is rotated one turn. The main-scanning start signal becomes valid to generate a memory read signal after the sub-scanning start signal changes from the "L" level to the "H" level.
In the color-image forming apparatus in which latent images are formed or transferred by superimposing a plurality of colors, the formation or the transferring of latent images are repeated a plurality of times. FIG. 24 illustrates an example in which the color-image forming apparatus forms or transfers latent images twice. In this example, the sub-scanning start signal in relation to the first rotation of the photosensitive member is generated somewhat before the corresponding main-scanning start signal, while the sub-scanning start signal in relation to the second rotation is produced slightly after the corresponding main-scanning start signal.
FIG. 24 reveals that the main-scanning start signal 1 becomes valid, since the sub-scanning start signal for the first rotation is generated slightly before the main-scanning start signal 1, so that the memory read signal corresponding to the first line of an image can be synchronized with the main-scanning start signal 1. Accordingly, upon counting the video CLK "n" times after the main-scanning start signal 1, as shown in FIG. 24, the memory read signal corresponding to the first rotation is generated.
In contrast, the sub-scanning start signal corresponding to the second rotation is generated later than that of the first rotation due to rotational fluctuations of the photosensitive member.
Since the sub-scanning start signal in relation to the second rotation is generated slightly after the main-scanning start signal 1, the memory read timing corresponding to the first line of an image is unfavorably synchronized with the main-scanning start signal 2 without detecting the main-scanning start signal 1. Accordingly, the memory read signal for the second rotation is generated after the video CLK has counted "n" times in synchronization with the main-scanning start signal 2.
Thus, one line of offset is generated between the memory read signal for the first rotation and that for the second rotation. As a consequence, in recording image data, which is read from the memory in response to the memory read signal, on the respective lines of the photosensitive member, the first line of the first rotation, which should be aligned with the first line of the second rotation, unfavorably matches the second line of the second rotation, thereby causing colors to shift.
According to the foregoing description, in conventional color-matching techniques, the rotational speed of, for example, a photosensitive member, changes due to load variations or a backlash of a driving transfer gear, etc., which further causes variations in the phase difference between the sub-scanning start signal and the main-scanning start signal. As a result, the positions at which the recording of the images of the respective colors is started are offset by one line or more.